4 Answers
4

The Shulchan Aruch in O.C. 260:1 says it is a mitzvah to trim ones nails on erev Shabbos. The Rema (ibid) adds that one should not trim them consecutively. Although the Taz 260:2 (based on the תשב"ץ and the Ariza"l) says one need not be concerned about this, the Magen Avraham 260:1 says that nevertheless one should be careful. This is echoed by the Shulchan Aruch HaRav 260:3, and Mishneh Berurah 260:8.

The Mishneh Berurah (260:6) mentions that it is the practice not to trim one's nails on Thursday. It is also mentioned there that there are authorities who say one should not trim one's finger nails on the same day as they trim their toenails.

The Mishneh Berurah also cites the Gemara (See Niddah 17a, Moed Katan 18a) which says one who burns their nail trimmings is a Chasid (pious person), one who buries them is a Tzadik (righteous person), and one who trows them out (haphazardly) is a Rasha (wicked person). It proceeds to explain that it is sufficient if one sweeps from their original location there is no need for concern, but cites the Elya Rabba that only when they have been removed from the original room is it considered a change of location (ibid 5).

I have not seen, to my recollection, any mention that one should refrain from trimming one's nails at night, but have seen one should not do so on Rosh Chodesh (Be'er Heitev 260:2 citing the Will of R. Yehuda haChasid) and that one should only trim one's fingernails on erev Shabbos or erev Yom Tov (Be'er Heitev ad loc).

There is a practice to trim ones fingernails and toenails on Erev Shavuos, since this is the practice of women prior to ritual immersion and on Shavuos Am Yisrael is, as it were, the kallah (bride) of Hashem.

If I recall correctly one of the main commentaries (Magen Avraham maybe?) says not to worry about the Rama's consecutive thing.
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ShalomApr 21 '10 at 16:27

Yirmeyahu, the Mishnah Berurah says there that just sweeping the nails away from the place where you cut them is good enough.
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YahuApr 21 '10 at 20:16

Shalom, I wasn't analyzing it halachicly, strictly speaking, since the questioner was asking about the basis for such ideas. Yahu-Saying one should sweep them up is sufficient doesn't negate that the Gemara says it is preferable to bury them and even better to burn them.
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YirmeyahuApr 22 '10 at 3:27

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Sometimes when answering a "what's the basis for Practice X" question, it can give the impression (certainly to some random online reader) that everyone agrees that one must keep Practice X all the time, so I added the dissenting opinion. See for instance www.yoatzot.org/question.php?id=5908
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ShalomApr 22 '10 at 12:23

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I agree with Shalom. It would probably be good, when answering such a question, to include something like, "Here are sources for the practices you describe. Of course, there are those who disagree."
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Isaac MosesApr 22 '10 at 13:24

My understanding is that we do not do in life as we do in death, like not sewing while clothes are on the body etc. WHen a Jewish person passes away, during the Tahara process, the nails are cleaned and even cut. This is in a consecutive order for ease of purpose. In life, doing differently, we stagger the cutting. My understanding is that this is a tradition and not a law! Many communities do not hold by this in any way.

Gary, welcome to Mi Yodeya, and thanks very much for contributing this answer! You could make it even more valuable by editing in a source for your understanding. Also, please consider registering your account, which will give you access to more of the site's features.
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Isaac MosesJan 21 '13 at 14:55

R' Herschel Schachter holds that this is neither Kabbalah nor halacha. Rather it's based on safety-that one should not accidentally cut oneself. These practices were recommended back in the days when people used knives which is not as safe as scissors or clippers.
Listen here about 22 minutes into the shiur.

Why the downvote? The OP has sourced his claim.
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Double AA♦Feb 4 '14 at 17:28

@DoubleAA me thinks somebody didn't like Rav Schachters opinion, he told me once that you dont need to wash netilas yadayim if i touched a covered part of my body, because we shower nowadays.
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MefareshMar 20 at 10:04

I've heard of all of the above, with two exceptions. I've heard that one should not cut his nails on Saturday night. Presumably this would be after Tzeith HaKochavim, since one is prohibited from cutting nails on Shabbath anyway. Also, I heard that one should not cut one's nails on Thursday (not Wednesday), because they might begin to grow on Shabbath. Why this is inherently a problem, I couldn't tell you.

However, I just read an interesting question posted on another website, which is related. It asks for a source for the "rule" that one should not cut one's nails "midweek" so that one is not tempted to cut them again on Shabbath. This gives credence to both the Thursday and the Wednesday opinions (assuming they are conflicting opinions and not merely different recollections of the same Halachah), and a viable reason. Basically, if one is concerned that one's nails are too long (and it's Wednesday or Thursday), then it is possible that one will be concerned again on Shabbath once they have started to grow long again, and that one might forgetfully cut them in violation of Halachah. It's a bit of stretch, perhaps, but it makes some sense. Why put yourself in a position to stumble? It's like Lifnei 'Iver for oneself even more so than a Geder.